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array_diff_ukey() is a very useful function in PHP. It allows us to compare the keys of two arrays based on user-defined comparison functions, and then return an array that contains key-value pairs that exist in the first array but not in the second array. While this function is very intuitive, the behavior can be confusing in some cases, especially when dealing with duplicate keys in an array.
The basic syntax of the array_diff_ukey() function is as follows:
array_diff_ukey(array $array1, array $array2, callable $key_compare_func) : array
$array1 : The first input array.
$array2 : The second input array.
$key_compare_func : a user-defined callback function for comparing two keys. The function accepts two parameters and must return an integer value indicating the relationship between the two keys.
When using array_diff_ukey() to compare the difference between key-value pairs, PHP will traverse the keys of the two arrays and judge whether the keys are equal through the comparison function provided by the user. When duplicate keys are present, PHP's behavior may not be exactly what we expect.
When the input array contains duplicate keys, array_diff_ukey() compares the value of each key with the key of the second array for comparison. In particular, if there are the same keys in the two arrays, but the keys are in different positions in the array, PHP compares based on the last occurrence of the key, ignoring the repeated keys that appear before.
When there are multiple duplicate keys in the array, array_diff_ukey() performs comparisons based on the last occurrence of key. For example, suppose we have the following two arrays:
$array1 = [
1 => 'apple',
2 => 'banana',
2 => 'orange',
3 => 'pear'
];
$array2 = [
2 => 'grape',
4 => 'watermelon'
];
In this example, key 2 is repeated in $array1 . Compare these two arrays by calling array_diff_ukey() :
$result = array_diff_ukey($array1, $array2, function($key1, $key2) {
return $key1 - $key2;
});
Although key 2 appears twice, array_diff_ukey() compares it with the last occurrence of 2 (i.e. the key corresponding to orange ) and compares it with key 2 in $array2 . As a result, key 2 will be considered the same, so the key will not appear in the result array.
If the logic of the comparison function is complex and involves certain comparison rules, array_diff_ukey() will still be compared based on the last occurrence of each key, although this comparison method may not be what we ideally think of. At this time, programmers can consider disposing of duplicate keys during the preparation stage of the array to avoid unnecessary confusion.
Suppose we want to process the array to ensure that the duplicate keys do not affect the comparison results of array_diff_ukey() . We can first use the array_unique() function to remove duplicate keys, and then perform the difference calculation.
$array1 = [
1 => 'apple',
2 => 'banana',
2 => 'orange',
3 => 'pear'
];
$array2 = [
2 => 'grape',
4 => 'watermelon'
];
// Remove duplicate keys
$array1 = array_unique($array1, SORT_REGULAR);
$result = array_diff_ukey($array1, $array2, function($key1, $key2) {
return $key1 - $key2;
});
In this example, we first use array_unique() to remove duplicate keys in $array1 to ensure that the key value comparison is more accurate.
array_diff_ukey() is a very powerful array difference function in PHP, but its behavior can be confusing when faced with repeated keys. PHP compares using the last value of each key, which means that duplicate keys are ignored. To avoid the impact of duplicate keys on the results, developers can ensure that the comparison operation can be performed as expected by deduplication of the array before using array_diff_ukey() .
